Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device relies on a transmitter (TX) and a receiver (RX), typically connected to a common antenna, for sending and receiving data and control signals over the wireless network. In order to use the common antenna, a multiplexer may be included to electrically connect the common signal path to the input of the receiver and to the output of the transmitter, so that the transmitter is able to send signals on a transmit frequency and the receiver is able to receive signals on a different receive frequency with negligible mutual interference between the transmit and receive signals. The multiplexer may be identified by different names depending on the number of signal paths to which the multiplexer is connected. For example, a duplexer connects two different signal paths, a triplexer connects three different signal paths, and a quintplexer connects five different signal paths. When the multiplexer connects the common antenna to the transmitter and receiver, the multiplexer is commonly called a transmit/receive antenna duplexer.
An antenna duplexer provides the necessary coupling, while preventing the transmit signal generated by the transmitter from being coupled from the common antenna back to the input of the receiver and overloading the receiver. Generally, the duplexer includes two band-pass filters having different passbands for filtering the transmit and receive signals, respectively, thus preventing or reducing interference between the transmit and receive signals. The filters are connected in parallel at the common antenna, and need to have sufficient rejection for the opposite band.
The antenna duplexer has three ports. The first port is connected to the antenna, the second port is connected to the transmitter and third port is connected to the receiver. Transmit and receive signals are assigned to different frequency bands, referred to as the Tx frequency band and the Rx frequency band, respectively. However, the transmit and receive signals co-exist at the common antenna, as discussed above. Thus, for the transmission path, the duplexer suppresses all signals outside the Tx frequency band to prevent interference with signals within the Tx frequency band sent to the antenna. Likewise, for the reception path, the duplexer suppresses all signals outside the Rx frequency band to prevent interference with signals within the Rx frequency band sent to the receiver from the antenna. Hence, the duplexer includes two frequency selective RF-filters, one filter for the Rx frequency band and the other filter for the Tx frequency band. Both RF filters are electrically connected to the common antenna port. In order to prevent the impedance of one of the RF filters to degrade the antenna side impedance of the other RF filter, an additional matching circuit is included.
The transmitters and receivers may be implemented in various types of wireless network, according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting (uplink or TX frequency band) and receiving (downlink or RX frequency band) signals. For example, UMTS Band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS Band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS Band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS Band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, an antenna duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
Demand for smaller, less expensive and more efficient portable communication devices is significant. Therefore, reducing size and weight of portable communication devices, as well as reducing fabrication costs and increasing product yield, are priorities. For example, there is demand for the band-pass filters of duplexers in portable communication devices to be smaller, to consume less power, to have improved performance characteristics (such as lower insertion loss and higher out-of-band attenuation), and to operate at higher frequencies. Such duplexers may include resonators for filtering the transmit and receive signals, such as a thin film bulk acoustic resonators (FBARs). However, design and fabrication are difficult, e.g., due to passband and stopband requirements of the corresponding receive and transmit band-pass filters, and matching circuit requirements between the band-pass filters and the antenna.